It is a goal of many prior art illumination systems used in projection display to provide a uniform light source having a substantially uniform intensity at and across a planar target location typically normal to the beam direction of the light source. In most instances this target location is rectangular in shape.
Various patents have addressed this problem by attempting to mix the light in light pipes or in free-space to later evenly spread the mixed light on a desired target homogeneously across it. For example, U.S. Pat. No. 5,098,184, incorporated herein by reference, issued Mar. 24, 1992 in the name of van den Brandt, et al. discloses a system wherein light directed from a light source captured by a parabolic reflector is redistributed such that the redistributed light arrives at its target with a more homogenous intensity distribution across the target region. As van den Brandt et al describe in their specification, “A parabolic reflector has of itself the advantage that it captures a large portion of the source radiation. However, the radiation beam formed by this reflector has an inhomogeneous distribution of the illumination intensity. It is true that a more homogeneous distribution can be obtained by arranging the radiation source outside the focal point of the parabola, but then the beam has a large angular aperture so that the projection lens system must have a large numerical aperture. The illumination intensity distribution can be made more homogeneous by using the lens plates in an illumination system with a parabolic reflector.” Prior art FIG. 1 illustrates a system shown in the van den Brandt et al. patent, wherein two arrays of lenses are arranged in series such that the first array separates a diverging incoming beam into plural sub beams, and wherein the second array in series with a lens mixes light received from the first array for later focusing onto a target plane. Although this system appears to function adequately, there is a need for a much more compact structure having fewer components and which is less costly to manufacture.
U.S. Pat. No. 5,909,316, incorporated herein by reference, issued Jun. 1, 1999 in the name of Watanabe is in many respects similar to the system of van den Brandt, however Watanabe utilizes a transmissive curved lens element with a lenslet array to mix or homogenize incoming light. The curved lens array conveniently both converges and separates a received single beam of light into a plurality of converging beams. This is in some respects an advance over van den Brandt's system. Notwithstanding, Watanabe's system, arranged in a transmissive configuration is complex and requires lenses.
It is an object of this invention to provide an element that can both capture light from a light source and reflect the captured light into a plurality of sub-beams for further processing in an imaging system.
It is a further object of this invention to provide a compact, folded, light diffuser having at least one array of mirrors for folding beams of light to be mixed, thereby providing a mixing of incoming light making the intensity more uniform over a desired target surface.
It also an object of this invention to provide a very inexpensive and compact light system for homogenizing received light and for imaging the homogenized light more uniformly across a rectangular image plane.